The ability of macrophytes to indicate pressures in rivers was assessed by comparing metrics for nitrate (NO3), ammonia (NH4), soluble reactive phosphorus (SRP), dissolved oxygen saturation (DO), pH (PH) and siltation (SUBS) with direct estimates of the pressures at 810 sites in the Republic of Ireland, supplemented with General Linear Models (GLMs). The bivariate and rank correlation coefficients using the full data range and the first and fourth quartiles of the river pressures varied between 0.22 and − 0.39 for NO3 and DO; they were smaller or not significant for the other four metrics. The GLMs provided evidence for an independent association between NO3 and the nitrate concentration and SUBS and ammonia, indicating some specificity for these metrics. Discriminating sites in the first and fourth quartiles produced Type II errors between 37 (PH) and 69% (NH4), with a mean of 50. As the pressure-impact relationships are not precise enough (low correlation coefficients) that evidence from a single macrophyte metric is reliable, combining the metric with evidence from other biological groups at one site or from three or more sites may be the most useful approach.